1. Field of the Invention:
This invention relates to a method for driving liquid crystal display devices and more particularly to a driving method for causing at least two liquid crystal display devices to simultaneously give the same display.
2. Description of the Prior Art:
Among the data imprinting devices for cameras, devices of the kind using liquid crystal display devices have increased in number during recent years. This tendency has resulted from the fact that the use of liquid crystal displays permits reduction in the thickness of the data imprinting device and also reduces cost. The basic arrangement of such a data imprinting device for a camera is as shown in FIG. 1 of the accompanying drawings. Referring to FIG. 1, the illustration includes a camera 1; a data imprinting device 2 which is provided on the back lid of the camera 1; a film 3; and a printed circuit board 4 on which an LSI 5 including a liquid crystal driver is mounted. On the printed circuit board 4 are further provided a transmission type liquid crystal display element (hereinafter referred to as LCD) 6 for data imprinting and a reflection type liquid crystal display element 7 for outside monitoring. The display elements 6 and 7 are respectively mounted on the board 4 through liquid crystal retainers 8 and 10 and elastic condcutors 9 and 11. The illustration further includes a lamp 12 which is a data imprinting light source, such as an LED or the like, and which illuminates the imprinting LCD 6 via apertures 13 and 14, a lens 15 and a light transmitting hole 4A provided in the printed circuit board 4. With the LCD 6 illuminated in this manner, the data displayed on the LCD 6 is imprinted on the film 3 in a shadowgraph like manner. The external monitor LCD 7 shows the same data as that of the LCD 6 to the outside by means of a reflection light through a display window provided in the back lid.
FIG. 2 shows the circuit arrangement of this device. A liquid crystal driver 5A is included in the above LSI 5 and is of the static driving type. Segment signal terminals "b" drive segments of the monitor LCD 7. Some of or all of the segment signal terminals "b" are also connected in parallel to the imprinting LCD 6. Data which is identical with a part of or the whole of the data displayed by the LCD 7 is also concurrently displayed by the LCD 6. This method of connection is considered to be the best method for reducing the number of terminals required for the liquid crystal driver 5A and for reducing the space required for wiring patterns.
A common signal terminal "a" is connected in common to the LCD 6 and the LCD 7. With the circuit arranged in this manner, when a segment 7-1-a of the monitor LCD 7, for example, is lit, a corresponding segment 6-1-a of the imprinting LCD 6 is also lit. When another segment 7-1-g of the LCD 7 is not lit, a corresponding segment 6-1-g of the LCD 6 is also not lit. However, since the LCD 7 is of the reflection type while the LCD 6 is of the transmission type, their displays are in a positive-and-negative reversed relation. Reference numeral 16 identifies a power supply; a transistor 17 lights up the light source 12; and a control circuit 5B controls the timing for a lighting up action.
FIG. 3 is a sectional view showing the structural arrangement of the above imprinting LCD 6. The illustration includes polarizing plates 18 and 19 which have polarizing characteristics, as indicated by arrows A and B; glass plates 20 and 21; a common electrode 22; a segment electrode 23; insulting films 25 and 26; sealants 27 and 28; and a metal mask 24 which is obtained by vapor depositing nickel or the like. The metal mask 24 includes light transmitting parts (or holes) 24a while the remainder 24b of the mask 24 has a light shielding property as shown in FIG. 4. The light transmitting parts 24a are formed in shapes according to the figures to be displayed. A liquid crystal layer 29 is sealed up in between the glass plates 20 and 21. The layer 29 is in many cases prepared by adding a dichromatic dye to a TN type liquid crystal. The light which is emitted from the light source 12 becomes an approximately parallel pencil of rays passing through the above aperture and the lens 15 and is going toward the film 3 passing through the above-stated parts 18, 20, 22, 24, 25, 29, 26, 21 and 19, 107 after another. However, since the polarizing plates 18 and 19 have the same polarizing direction, the portion of the light impinging on the non-conductive segments of the liquid crystal, i.e. liquid crystal portions having 90.degree. rotatory polarization characteristics are not allowed to pass through them and thus fail to reach the film 3. Conversely, a conductive liquid crystal portion which has received power to lose the rotary polarizing property allows a portion of the illumination light to pass therethrough and to reach the film 3. The conventional imprinting LCD 6 thus energizes the segments which are to be used for forming display figures to allow the light to pass through them for imprinting the figures or letters, etc. on the film 3. However, since the polarizing direction A,B of one polarizing plate 18 or 19 is the same as that of the other and the light shielding performance is dependent on the 90.degree. rotary polarization of the liquid crystal molecules, the conventional LCD has not enhanced the constrast of the display, or the ratio of the transmission factor of an energized portion to that of a non-energized portion, by an insufficient absolute light shielding degree. This has resulted in poor quality letters and figures of data to be imprinted on the film 3.
To compensate for this drawback, the constrast has been increased by adding the dichromatic dye to the liquid crystal. However, this method not only causes an increase in cost but also results in a decrease response. To solve this problem, it has been contrived to use an LCD 6' which is arranged as shown in FIG. 5 for imprinting. In FIG. 5, the same component parts as those shown in FIG. 3 are identified by the same reference numerals. The arrangement of FIG. 5 differs from that of FIG. 3 in that polarizing directions C and D of polarizing plates 18' and 19' differ from each other by 90 or 270 degrees. By virtue of this difference, a portion of the illumination light impinging upon non-energized segments having 90.degree. rotary polarizing property is allowed to reach the film by passing through the polarizing plate 19' while energized segments do not allow the light to pass through them. In other words, there is obtained a relation which is the reverse of the LCD 6 of FIG. 3. In accordance with this method, the light shielding performance can be greatly enhanced for improved constrast since it is no longer dependent on the 90.degree. rotary polarization of the liquid crystal molecules and the light is prevented from passing through by virtue of the difference in the polarizing direction C,D between the polarizing plates 18' and 19'. Therefore, addition of the dichromatic dye is no longer necessary. This method thus prevents the cost of the display device from increasing and the response thereof from decreasing.
FIG. 6(a) shows an energized state of and a display made by the LCD 6 shown in FIG. 3. FIG. 6(b) shows the energized state of the LCD 6' shown in FIG. 5. Since they have polarizing directions A,B and C,D conversely arranged as mentioned above, the relation between energized segments and non-energized segments of the LCD 6 is reverse to that of the LCD 6' in displaying the same figures. In these drawings, blackened segments represent segments energized segments and voidly shaped segments non-energized segments. In the LCD 6 shown in FIG. 6(a), the segments to be displayed are energized to make them light transmissive. Whereas, in the case of the LCD 6' shown in FIG. 6(b), the segments to be displayed are not energized and are made light transmissive.
It is preferable for the data imprinting device 2 of the camera 1 to have good contrast both for display and imprinting without adding a dichromatic dye as with the LCD 6' described above. However, in that case, with the driving circuit 5A of FIG. 2, which is arranged to use the segment terminal b and the common terminal a in common for both the imprinting LCD 6 and the monitor LCD 7, there arises the following problem:
More specifically, when the segment driver 5a does not energize the segments of the imprinting LCD 6 which are to be used for a display and energize the segments which are not to be used for the display, the reflection type monitor LCD 7, which is of the TN type, has its non-energized segments become light transmission regardless of the polarizing directions C, D of the upper and lower polarizing plates 18', 19' and thus the color of these segments becomes barely discernible from that of the reflecting plate. As a result, the display by the LCD 7 becomes as shown in FIG. 6(b).
It is an object of this invention to provide a data imprinting device 2 in which the LCD's 6,7 make a high contrast display as described above with reference to FIG. 5 by a simple method without incurring the above-stated drawback of the prior art device.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.